Water-retaining support for plants and plant body-growing water-retaining material

ABSTRACT

A water-retaining support for plants comprising a hydrogel-forming polymer (A) having a calcium ion absorption of 0-100 mg per 1 g of the dry weight thereof, having a chlorine ion content of 0.07-7 mmol per 1 g of the dry weight thereof and having a water absorption magnification in ion-exchanger water at 25° C. of 1.0×10 1  to 1.0×10 2 ; or a plant body-growing water-retaining material comprising a molded product of a mixture of such a polymer (A) and a plant body-growing support (B). The water-retaining support for plants and the plant body-growing water-retaining material are those which have an excellent water-retaining property and they substantially do not cause inhibition of root generation or inhibition of root elongation.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of International application No.PCT/JP99/06187, filed Nov. 5, 1999, which in turn claims priority toJapanese patent Application No. 10/316440, filed on Nov. 6, 1998,Japanese patent Application No. 11/20394, filed on Jan. 28, 1999,Japanese patent Application No. 11/50305, filed on Feb. 26, 1999, andJapanese patent Application No. 11/290552, filed on Oct. 13, 1999.

TECHNICAL FIELD

The present invention relates to a water-retaining support (or carrier)for plants and a plant body-growing water-retaining material which cansupport or hold a plant at the time of the growth of the plant and canalso function as a source for supplying water to the plant. Morespecifically, the present invention relates to a water-retaining supportfor plant and plant body-growing water-retaining material which cansupply water to a plant without inhibiting the growth of the plant, whenthe support is used as a water-retaining support for fluid seeding (orseeding using a fluid), farm cultivation, field (or bare ground)cultivation, virescence (or greening) engineering, etc.

BACKGROUND ART

Polycarboxylic acid-type highly water-absorbing resins, especiallypolyacrylic acid-type polymers, which have been used in a large quantityfor diapers, menstrual goods, etc., are also used in the field ofagriculture due to their inexpensiveness and excellent water-retainingability.

For example, hydrogels of the polyacrylic acid-type polymers have beenused as a support for fluid seeding; or a water-retaining support forvirescence engineering, water-saving cultivation, or cultivation onsandy soil, by utilizing their water-retaining ability.

However, it has been recognized that the conventional polyacrylicacid-type hydrogels affect the growth of a plant, and particularly, theycause a marked inhibition of the root origination and root elongationwhen the hydrogels are used in an amount exceeding their appropriateamount (Kazuo Kawashima, et al., “Influences on The Early Growth ofVegetables by a Super Absorbent With Cross-linked Forms ofPolyacrylate,” Sand Dune Research Institute, 31(1), 1-8, 1984).

Particularly, when the conventional polyacrylic acid-type hydrogel isused as a support for tissue culture, a support for fluid seeding, and asupport for virescence engineering, a plantlet, seed, etc., of a plantis caused to directly contact the high-concentration polyacrylicacid-type hydrogel, and therefore its root origination and rootelongation are markedly inhibited, whereby the use of the polyacrylicacid-type hydrogel is severely restricted.

It has also been recognized that, in a case where the conventionalpolyacrylic acid-type hydrogen is used as a water-retaining support forfarm or field cultivation, the elongation of the root is inhibited whenthe concentration of the polymer in the vicinity of the root isincreased so as to enhance the effect of the water-retaining support.

As an example of the phenomenon such that the above-mentioned hydrogelcomprising a polyacrylic acid-type resin markedly inhibits the growth ofa plant, there has been reported an experiment wherein distilled waterwas absorbed into a crosslinked sodium polyacrylate so as to form ahydrogel, and the thus obtained hydrogel was caused to contact seeds ofcucumbers and kidney beans for respective periods of time (3, 6, 9, 12,24 and 48 hours), and then the states of the germination and rootorigination of the seeds were observed (Kazuo Kawashima, et al.,“Influences on The Early Growth of Vegetables by a Super Absorbent WithCross-linked Forms of Polyacrylate,” Sand Dune Research Institute,31(1), 1-8, 1984).

As a result of such experiments, it has been reported that the growth ofroots was markedly suppressed in the case of cucumber seeds, when theyare caused to contact the hydrogel for 36 to 48 hours, and that theinhibition of root growth was also observed in the case of kidney beans.Further, it has been reported that the α-naphthylamine-oxidizing abilityof the root was markedly reduced when the root is caused to contact theabove-mentioned hydrogel for 5 hours or more. In this report, suchgrowth inhibition and functional hindrance are presumably attributableto a fact that the plant cannot effectively use the water contained inthe hydrogel.

On the other hand, it has been reported that, when rice seeds were sownon a hydrogel which had been prepared by causing crosslinked sodiumpolyacrylate to absorb water, and then the process of the rootorigination was observed, serious hindrance in the root origination wasrecognized (Yorio Sugimura, et al., “Utilization of HighWater-Absorptive Polymers as Greening Engineering Material,” Techniquesof Virescence Engineering, 9(2), 11-15, 1983). In this report, however,no hindrance in the root origination was observed when the hydrogel wasdialyzed with tap water, but the recovery of the root growth was notobserved even when the hydrogel was dialyzed with distilled water. Withrespect to this phenomenon, in this report, it is presumed that, whenthe hydrogel is washed or dialyzed with a weak electrolytic solutionsuch as tap water, the water-absorbing force toward the hydrogel wasweakened, and the migration of water from the gel to the root hair isfacilitated, to thereby solve the hindrance in the root origination.

There has also been reported an example wherein the degree of theelongation of soybean root was markedly inhibited in a soil which hadbeen mixed with a crosslinked sodium polyacrylate hydrogel, as comparedwith that in the case of a polyvinyl alcohol-type hydrogel (TomokoNakanishi, Bioscience & Industry, 52(8), 623-624, 1994). In thisreference, this phenomenon is presumably attributable to a fact that thewater in the sodium polyacrylate hydrogel is less liable to be utilizedby a plant.

As described above, it has heretofore been considered that theinhibition of the growth of a plant in a hydrogel comprising an alkalimetal salt of crosslinked polyacrylic acid is attributable to the factthat the water in the hydrogel is not effectively utilized by the plant.

An object of the present invention is to provide a water-retainingsupport for plants which has solved the above-mentioned problems of thehydrogel water-retaining

Another object of the present invention is to provide a water-retainingsupport for plants which has a water-retaining ability comparable tothat of the conventional polyacrylic acid-type hydrogel, and does notsubstantially cause an inhibition of root origination or of rootelongation.

A further object of the present invention is to provide a plantbody-growing water-retaining material which has a good water-retainingability and does not substantially cause inhibition of root originationor of germination of a plant.

As a result of earnest study, the present inventors have found that theeffect of a hydrogel is so strong that the inhibition of the rootelongation cannot be simply attributable to the effectiveness in theutilization of water in the hydrogel.

As a result of further study based on the above discovery, the presentinventors have also found not only that the calcium ion-adsorbingability in the hydrogel has an important effect on the inhibition ofroot origination or the inhibition of root elongation of a plant whichis in contact with the hydrogel; but also that, in combination with theabove-mentioned calcium ion-adsorbing ability, chlorine ions present inthe hydrogel also have an important effect on the inhibition of rootorigination or on the inhibition of root elongation of a plant which isin contact with the hydrogel.

DISCLOSURE OF INVENTION

The present invention relates to a water-retaining support for plantscomprising a hydrogel-forming polymer (A) having a calcium ionabsorption of 0-100 mg per 1 g of the dry weight thereof, having achlorine ion content of 0.07-7 mmol per 1 g of the dry weight thereof,and having a water absorption magnification in ion-exchange water at 25°C. of 1.0×10¹ to 1.0×10³; and to a plant body-growing water-retainingmaterial comprising a mixture of such a hydrogel-forming polymer (A) anda plant body-growing support (B). Herein, the “water-retaining support”refers to one in a “dry state” unless otherwise noted specifically. Whensuch a support is distributed or circulated on the market, etc., thesupport may also be in a “hydrogel” state wherein a part or the entiretyof the support retains water therein.

BEST MODE FOR CARRYING OUT THE INVENTION

The reason that the water-retaining carrier for plant according to thepresent invention has a water-retaining ability comparable to that ofthe conventional polyacrylic acid-type hydrogel but causes substantiallyno inhibition of root origination nor inhibition of root elongation of aplant, is not completely clear, but according to the present inventors'knowledge, it is presumed to be as follows.

As a result of experiments, described hereinafter, the present inventorshave found a fact that the conventional hydrogel comprising an “alkalimetal salt of crosslinked polyacrylic acid” selectively adsorbs heavymetal ions, mainly calcium ions.

In other words, according to the present inventors' experiments, it waspresumed that the conventional crosslinked polyacrylic acid-typehydrogel adsorbs ions (mainly comprising calcium ions) in agriculturalwater (such as well water, tap water, river water, and lake water) andthe plant suffers from deficiency of calcium ions; or the hydrogeldirectly adsorbs ions (mainly comprising calcium ions) from the plantbody through its roots, whereby the plant suffers from a deficiency ofcalcium ions.

As a result of further experiments, the present inventors have alsofound a fact that when the water-retaining support (or carrier) forplant according to the present invention comprising a hydrogel-formingpolymer (A) (having a specific calcium ion absorption amount) is incontact with a plant root, etc., the direct absorption of calcium ionsby the water-retaining support from the root based on such contact iseffectively suppressed by the chlorine ions present in thewater-retaining support. It is presumed that plant water-retainingsupport according to the present invention which not only has a specificcalcium ion absorption amount, but also has a specific chlorine ioncontent does not substantially cause the inhibition of the rootorigination nor the inhibition of root elongation on the basis of thecombination of the above-mentioned effects.

Hereinbelow, the present invention will be described in detail withreference to the accompanying drawings as desired. In the followingdescription, “%” and “part(s)” representing a quantitative proportion orratio are those based on weight, unless otherwise noted specifically.

(Water-retaining support)

The water-retaining support according to the present invention comprisesa hydrogel-forming polymer (A) having a calcium ion absorption (amount)of 0-100 mg per 1 g of the dry weights thereof, having a chlorine ioncontent of 0.07-7 mmol per 1 g of the dry weight thereof, and having awater absorption magnification in ion-exchange water of 1.0×10¹ to1.0×10³ (times) or more. In the present invention, the above-mentioned“calcium ion absorption”, “chlorine ion content” and “water absorptionmagnification in ion-exchange water” may suitably be measured, e.g., bythe following method.

(Measurement of Calcium Ion Absorbing Amount)

1 g of a dried water-retaining support is added to 1 L (liter) ofaqueous calcium chloride solution having a calcium ion concentration of200 mg/L. Then, the resultant mixture is left standing for 48 hours in aconstant-temperature bath (or thermostatic chamber) at 25° C., while themixture is stirred occasionally, to thereby cause the water-retainingsupport to absorb calcium ion while being swollen. The thus swollenwater-retaining support is separated from the supernatant, and thecalcium ions concentration in the remaining supernatant (excess amountthereof in the above-mentioned aqueous calcium chloride solution) isquantitatively determined by atomic absorption spectrometry (A mg/L).

At this time, in the measurement of the calcium ion analysis by theabove atomic absorption spectrometry, the following conditions arepreferably usable.

<Measurement conditions for atomic absorption analysis>

Atomic absorption spectrometer: trade name: AA-6500 Auto-System, mfd. byShimazu Corporation

Lighting condition: Ca #8

Electric current: 10 mA/0 mA

Wavelength: 422.7 nm

Slit width: 0.5 μm

On the basis of the thus determined value (A) of the calcium ionconcentration, the calcium ion absorption amount per 1 g of thewater-retaining support is obtained by the following formula. At thetime of the separation of the supernatant from the water-retainingsupport, there is a possibility that the non-crosslinked water-solublepolymer is dissolved in the supernatant, and therefore it is preferredto effect separation by ultrafiltration using an ultrafilter membranehaving a fractionation molecular weight of about 1,000 to 3,000.

Calcium ion absorption amount per 1 g of water-retaining support(mg/g)=200−A

When the calcium ion absorption amount measured by the above-mentionedmethod exceeds 100 mg per 1 g of the dry weight of the water-retainingsupport, calcium ion deficiency is liable to occur in a plant which isin contact with the water-retaining support, as shown in Examplesappearing hereinafter. In the present invention, the calcium ionabsorption is 100 mg or less, per 1 g of the dry weight of thewater-retaining support.

(Action of chlorine ions)

In the water-retaining support according to the present invention, thecontent of chlorine ions in the hydrogel-forming polymer (A) is 0.07-7mmol per 1 g of the dry weight thereof.

According to the present inventors' knowledge, it is presumed that thephenomenon that the chlorine ions present in the hydrogel according tothe present invention suppresses the adsorption of calcium ions from theinternal plant body due to the hydrogel by the following mechanism, onthe basis of the various experimental data obtained by the presentinventors.

Thus, it is considered that the absorption and desorption process ofcalcium ions by a plant is mainly effected physico-chemically, and isgoverned by the concentration gradient of calcium ions between theinside and outside of the plant body (for example, “An Outline of PlantDietetics” (Shokubutu Eiyogaku Taiyo) edited by Kikuo Kumazawa, p 118,Yokendo Co., Ltd., 1974 may be referred to). The calcium ions in theplant body are present in association with chlorine ions, nitrate ions,phosphate ions, hydroxide ions, etc., as counter ions. It is consideredthat calcium ions in association with the phosphate and hydroxide ionsare hardly water-soluble, and therefore do not participate in theabsorption and desorption process by the plant.

On the other hand, it is presumed that the calcium ions in associationwith the nitrate ions are water-soluble, but the nitrate ions which havebeen absorbed into the plant body are promptly reduced into nitrite ionsby nitrate reductase, and are further reduced into ammonium ions bynitrite reductase, and therefore they are not utilized as contour ionsfor calcium ions (for example, “Introduction to Life Science of Plants”(Shokubutu no Seimei Kagaku Nyuumon) written by Seiichiro Kamisaka, p138, Baifukan, Co., Ltd., 1991 may be referred to).

Accordingly, there can be made a presumption that the ions closelyrelated to the absorption and desorption process of calcium ions arechlorine ions, and under an electrically neutral condition, the entranceand exit of calcium ions are accompanied with the entrance and exit ofchlorine ions.

Herein, when the chlorine ion concentration in the external liquid ishigher than the chlorine ion concentration in the plant body, thetransport of calcium ions from the internal plant body to the outside ofthe plant body must be conducted against the gradient of the chlorineion concentration, whereby the desorption of the calcium ion from theplant body may be suppressed.

On the other hand, when the chlorine ion concentration in the externalliquid is lower than the chlorine ion concentration in the plant body,the desorption process of the calcium ions from the plant body may bepromoted.

Further, the absorption of calcium by the plant body (reversely to thecase of the desorption thereof) is promoted when the chlorineconcentration of the external liquid is higher than the concentration ofthe internal body, and such absorption is suppressed when the chlorineconcentration of the external liquid is lower than the concentration ofthe internal body.

That is, according to the knowledge of the present inventors, it ispresumed that calcium ions are liable to be accumulated in the internalplant body when the chlorine ion concentration in the external liquid ishigher than the chlorine ion concentration in the internal body liquid,and reversely, the calcium ions are liable to be desorbed to the outsideof the plant body when the chlorine ion concentration in the externalliquid is lower than the chlorine ion concentration in the internal bodyliquid.

The above-mentioned “presumptive mechanism” according to the presentinventors is supported by a fact that the chlorine ion concentration inthe internal plant body is considered to be about 7 mM (for example,Higinbotham, N. B., et al., Plant Physiol. 42, 37,1967 may be referredto). In other words, it is considered that according to the “presumptivemechanism” based on the experimental data obtained by the presentinventors, and on the description of the literature that “the chlorineion concentration in the internal plant body is about 7 mM”, thechlorine ion concentration in the exterior liquid body may preferably beat least about 7 mM or more in order to suppress the desorption ofcalcium from the internal plant body.

Usually, the chlorine ion concentration in agricultural water such aswell water, tap water, river water, and lake water to be used for thegrowth of plants is 1 mM is or less, which is much lower than thechlorine ion concentration in the plant body.

When such agricultural water is absorbed into the hydrogel according tothe present invention to thereby swell the hydrogel with the water, thechlorine ion content per 1 g of the dry weight of the hydrogel (“a”,mmol/g) required to cause the value of the chlorine concentration in thehydrogel to be a value which is at least not lower than the chlorineconcentration in the plant body (usually, about 7 mM) is expressed bythe following formula:

a=7×b/1,000

(wherein “b” denotes the water absorption magnification of thehydrogel).

Herein, the water absorption magnification (B) of the hydrogel supportwith agricultural water is a numerical value which is dependent on thechemical composition of the hydrogel and the salt concentration in theagricultural water. When the above value “a” is calculated by using theabove formula provided that the above value “b” is about 10 times toabout 1,000 times, the value “a” is about 0.07 mM/g to about 7 mmol/g.In other words, the “preferred chlorine ion content” (0.07-7 mmol) basedon the experimental data of the present inventors is also supported bysuch calculated values.

As described in Table 1 of [Examples] appearing hereinafter, in the caseof a hydrogel having a calcium ion absorption of 100 mg/g or more per 1g of dry weight thereof and containing no chlorine ion in the hydrogel(commercially available polyacrylic acid salt-type hydrogel described inComparative Example 6), remarkable growth inhibition in the root andstem were observed.

Further, in a case where the chlorine ion content in the polymer (A) iseven in the range of 0.07-7 mmol per 1 g of dry weight thereof, when thecalcium ion absorption exceeds 100 mg/g (Comparative Example 7), markedgrowth inhibition in the roots and stems were observed.

Further, even in a case where the calcium ion absorption is 100 mg/g orless, when no chlorine ion is contained in the gel (Comparative Examples1-5), marked growth inhibition in the roots and stems were observed.

On the other hand, even in the case of a hydrogel showing a low calciumabsorption (63 mg/g), when the chlorine ion concentration in thehydrogel exceeds 7 mmol/g (Comparative Example 8), not only theabsorption magnification was markedly lowered and the performance as awater-retaining support is lowered, but also marked growth inhibition inthe roots and stems were observed due to salt damage based on a largedifference in the chlorine ion concentration between the inside andoutside of the plant body.

As shown in Table 1 appearing hereinafter, in the case of thewater-retaining support for plant according to the present inventioncomprising a hydrogel-forming polymer which has a calcium absorption ofless than 100 mg/g, and has a chlorine ion content of 0.07-7 mmol/g(Examples 1-4), they suitably function as a water-retaining support forplant without inhibiting the growth of the roots and stems at all.

As described above, in the case of the calcium ion deficiency caused byhydrogels having a calcium absorption and a chlorine ion content bothoutside of the range according to the present invention (hydrogels basedon commercially available polyacrylic acid sodium salt have a highcalcium absorption and contain no chlorine ion, as described inComparative Example 6), the cell membrane structure is destroyed, andmany important functions (such as cell division) depending on themembrane structure are stopped or retarded, whereby the root elongationis markedly inhibited in view of the appearance thereof (with respect tothe details of such calcium ion deficiency, “Outline of Plant Nutritionscience” edited by Kikuo Kumazawa, p 118, Yokendo Co., Ltd., 1974 may bereferred to).

(Measurement of chlorine ion content)

0.2 g of a hydrogel-forming polymer in a dry state is immersed in 200 mlof ion exchange water, and left standing for two days. The resultantsupernatant is filtrated by a filter, and the chlorine ion concentrationin the filtrate is analyzed by an ion analyzer (Ion Analyzer IA-100,mfd. by TOA Electric Wave Industries (TOA Denpa Kogyo)). Based on thethus obtained chlorine ion concentration, the quantity of the chlorineions contained in 200 ml of the above ion exchange water is determinedby calculation, the resultant calculated value is treated as thequantity of chlorine ions in “0.2 g of dry hydrogel-forming polymer”.

At this time, in the above chlorine ion analysis by an ion analyzer, thefollowing conditions may suitably be used.

<Measurement conditions for ion analyzer>

Column: PCI-201S for anion (mfd. by TOA Electric Wave Industries) andCard Column PCI-201SG (mfd. by TOA Electric Wave Industries)

Solvent: Eluent Liquid for anion (mfd. by TOA Electric Wave Industries)

Column oven temperature: 40±4° C.

If the “chlorine ion content” measured by the above method is in therange of 0.07-7 mmol per 1 g of dry weight of water-retaining support,it is possible to suppress the “calcium ion deficiency” as shown inExamples appearing hereinafter.

(Measurement of water absorption magnification in ion-exchange water)

A predetermined amount (W₁ g) of a dried water-retaining support isweighed, then is immersed in an excess amount (e.g., a weight which isat least 1.5 times the expected water-absorption amount of theabove-mentioned water-retaining support) of ion-exchange water (havingan electric conductivity of 5 μS/cm or less), and is then left standingin a constant-temperature bath at room temperature (25° C.) for 2 days(48 hours) whereby the water-retaining support is swollen. An excessamount of water is removed by filtration, and thereafter the weight (W₂g) of the water-retaining support which has absorbed water to be swollentherewith is measured. Then, the water absorption magnification isdetermined by the following formula:

Water absorption magnification=(W ₂ −W ₁)/W ₁

At the time of the measurement of this water absorption magnification,it is preferred to measure the weights W₁ and W₂ by using a preciseelectronic balance (for example, LIBROR AEG-220, LIBROR EB-3200-D, mfd.by Shimazu Corporation etc.).

If the water absorption magnification measured by the above-mentionedmethod is less than 10 (times), it becomes difficult to sufficientlysupply water to a plant when a predetermined amount of thewater-retaining support is used. In the present invention, the waterabsorption magnification is 10-100, but the water absorptionmagnification may preferably be 30-900, more preferably 50-800.

When the water-retaining support according to the present invention isused in combination with water having a relatively low saltconcentration such as agricultural water, for example, the waterabsorption magnification of a hydrogel constituting the support may beimproved most effectively by introducing a dissociative ion group intothe gel so as to expand the molecular chains in the gel and tosimultaneously enhance the internal osmotic pressure in the gel.

(Hydrogel-forming polymer (A))

The hydrogel-forming polymer (A) constituting the water-retainingsupport according to the present invention refers to a polymer which hasa crosslinked or network structure, and has a property such that it canform a hydrogel by retaining water (in the inside thereof) on the basisof such a structure. Further, the “hydrogel” refers to a gel which atleast comprises a crosslinked or network structure comprising a polymer,and water (as a dispersion liquid) supported or retained by such astructure.

The “dispersion liquid” retained in the crosslinked or network structureis not particularly limited, as long as it is a liquid comprising wateras a main or major component. More specifically, the dispersion liquidmay for example be either of water per se, an aqueous solution and/orwater-containing liquid (e.g., a mixture liquid of water and amonohydric or polyhydric alcohol).

In the present invention, it is preferred to use a product obtained bycrosslinking a water-soluble or hydrophilic polymer compound, as theabove-mentioned polymer (A). Such a crosslinked polymer has a propertysuch that it absorbs water in an aqueous solution to be swollen, but isnot dissolved therein. The water absorption rate may be changed bychanging the kind of the above-mentioned water-soluble or hydrophilicpolymer and/or the density (or degree) of crosslinking thereof.

(Water-soluble or hydrophilic polymer compound)

Specific examples of the water-soluble or hydrophilic polymer forproviding a hydrogel constituting the water-retaining support accordingto the present invention may include: methyl cellulose, dextran,polyethylene oxide, polypropylene oxide, polyvinyl alcohol, poly(N-vinylpyrrolidone), poly(N-vinyl acetamide), polyvinyl pyridine,polyacrylamide, polymethacrylamide, poly(N-methyl acrylamide),polyhydroxymethyl acrylate, polyacrylic acid, polymethacrylic acid,polyvinylsulfonic acid, polystyrenesulfonic acid and their salts,poly(N,N-dimethylaminoethyl methacrylate), poly(N,N-diethylamnioethylmethacrylate), poly(N,N-dimethylaminopropyl acrylamide), and theirsalts, etc.

(Crosslinking)

As the method of imparting or introducing a crosslinked structure to theabove-mentioned polymer compound, there are a method wherein acrosslinked structure is introduced into the polymer at the time of thepolymerization of the monomer for providing the polymer; and a methodwherein a crosslinked structure is introduced to a polymer after thecompletion of the polymerization of the monomer. Each of these methodsmay be used in the present invention.

The former method (i.e., introduction of crosslinking at the time ofmonomer polymerization) may generally be conducted by utilizing thecopolymerization with a bifunctional monomer (or a monomer having threeor more functional groups). For example, such a method may be conductedby using a bifunctional monomer including: bis-(meth)acrylamides such asN,N-methylene bis-(meth)acrylates of polyhydric alcohols such as(poly)alkylene glycol, trimethylol propane, glycerin, pentaerythritol,and sorbitol; divinyl compounds such as divinylbenzene; allyloxyalkanessuch as tetraallyloxy ethane, and pentaerythritol triallyl ether; etc.,singly or as two or more species thereof.

The latter method (i.e., introduction of crosslinking after the monomerpolymerization) may generally be conducted by forming a crosslinkbetween molecules by utilizing light, an electron beam, γ-rayirradiation, etc.

Further, the latter method may also be conducted by crosslinking apolymer, e.g., by using, as a crosslinking agent, a multi-functionalmolecule having a plurality of functional groups (such as isocyanategroup, and glycidyl group) which are capable of being bonded to afunctional group (such as carboxyl group, and amino group) in thepolymer. In this case, it is possible to use a crosslinking agent whichis similar to those to be used for surface crosslinking of particles asdescribed hereinafter.

In the present invention, the above-mentioned water absorption rate ofthe (A) is dependent on the above-mentioned crosslinked structure,particularly the density of crosslinking of the polymer. In general, asthe crosslinking density becomes lower, the water absorption rate tendsto increase.

In the former method, the crosslinking density may arbitrarily becontrolled, e.g., by changing the copolymerization ratio of thebifunctional monomer. In the latter method, the crosslinking density mayarbitrarily be controlled, e.g., by changing the quantity of irradiationsuch as light, an electron beam, and γ-ray irradiation.

In the present invention, the crosslinking density may preferably be inthe range of about 0.01 mole % to about 10 mol %, more preferably about0.05 mol % to about 5 mol %, in terms of the ratio of the moles of thebranching point to the moles of all the monomer. Alternatively, when thecrosslinked structure is introduced by the former method (introductionof crosslinking at the time of polymerization), the crosslinking densitymay preferably be in the range of about 0.005 wt. % to about 3 wt. %,more preferably about 0.01 wt. % to about 2 wt. %, in terms of thecopolymerization weight ratio of the bifunctional monomer to all themonomers (inclusive of the bifunctional monomer per se).

When the crosslinking density exceeds about 10 mol %, the waterabsorption magnification of the polymer (A) according to the presentinvention is decreased, whereby the effect of the polymer (A) as thewater-retaining support is decreased. On the other hand, when thecrosslinking density is below about 0.01 mol %, the polymer (A) becomesmechanically weak, and the handling thereof becomes difficult.

The crosslinking density (molar ratio of the branching points withrespect to all the monomer) may be determined quantitatively, e.g., by¹³C-NMR (nuclear magnetic resonance absorption) measurement, IR(infrared absorption spectrum) measurement, or elemental analysis.

Further, in the polymer (A) constituting the water-retaining supportaccording to the present invention, it is also possible to obtain abetter balance between a high water absorption magnification and a highmechanical strength in the polymer (A) by making the crosslinkingdensity higher in the vicinity of the surface than that in the insidethereof (i.e., by introducing so-called “surface crosslinking”). In suchan embodiment, the portion having a relatively high crosslinking densityin the vicinity of the surface may mainly contribute to the highmechanical strength (and to an improvement in the non-stickiness betweensupport particles), while the portion having a relatively lowcrosslinking density in the inside may mainly contribute to the highwater absorption magnification. Thus, it becomes easy to realize apreferred mechanical strength and a preferred non-stickiness between theparticles substantially without decreasing the water absorptionmagnification.

In view of the balance between the water absorption magnification andmechanical strength, the ratio (Ds/Di) of the highest crosslinkingdensity Ds in the vicinity of the surface to the lowest crosslinkingdensity Di in the inside of the particle in the above-mentionedembodiment may usually be about 2 to 100, more preferably about 5 to 100(particularly, about 10 to 100).

The crosslinking density in the vicinity of the surface and that in theinside of the particle may be measured by determining the ratio of thepresence of the crosslinking agent in the vicinity of the surface andthat in the inside of the particle, e.g., according to a local analysistechnique such as electron spectroscopy for chemical analysis ESCA(XPS), electron probe microanalysis EPMA, attenuated total reflection(ATR), or secondary ion mass spectrometry SIMS (time-of-flight SIMS(TOF-SIMS), etc.).

In the water-retaining support for plants according to the presentinvention, when the polymer (A) constituting the support has a highmechanical strength, it becomes easy to keep appropriate voids (orcavities) between the individual support particles, and the presence ofthe voids may further improve the capability of the support to supplyoxygen to the root of a plant.

In the present invention, the method of introducing the surfacecrosslinking to the polymer (A) is not particularly restricted, and itis possible to use, e.g., various kinds of known methods (or acombination of two or more of such methods).

Particularly, when the polymer (A) in the present invention has acarboxyl group bonded to the polymer chain thereof, it is preferred touse a method wherein a crosslinking agent having at least two functionalgroups capable of reacting with the carboxyl group is used to crosslinka portion in the vicinity of the surfaces of fine polymer particles.Examples of such a crosslinking agent may include: polyglycidylcompounds having 2-10 epoxy groups per one molecule thereof such asethylene glycol diglycidyl ether, glycerol diglycidyl ether, andpolyglycerol polyglycidyl ether; polyhydric alcohols containing 2-10carbon atoms such as glycerin and ethylene glycol, alkylene carbonatescontaining 2-10 carbon atoms in the alkylene group thereof, polyamineresins such as polyamide polyamine epichlorohydrin resin, and polyamineepichlorohydrin resin (molecular weight: about 2×10²-5×10⁵), polyvalentisocyanate compounds (as described in JP-A Sho. 59-189103), polyvalentazetidinium compounds (as described in JP-A Hei. 6-287220), etc. Thesecrosslinking agents may used singly or in combination of two or morekinds thereof as desired. Among these, in view of the possibility of acrosslinking reaction at a relatively low temperature, polyglycidylcompounds and polyamine resins may preferably be used.

In the present invention, the shape, size, etc., of the polymer (A) andaggregates of particles are not particularly limited. For example, it ispossible to use those in the form powder, granules, lumps (blocks),etc., and it is possible to use those in the sizes of 1 μm to severalcentimeters. Depending on the purpose of using these materials, it ispossible to appropriately select the shape, size, etc. For example, whenthe polymer of polymer (A) is used singly, the particle size thereof maypreferably be relative large, and the polymer may preferably be in theform of powder of 300-5,000 μm. When it is used in combination withanother carrier material, the polymer may preferably be those in theform of powder of 5-1,000 μm.

As the technique for crosslinking the surface of a polymer (A) with theabove crosslinking agent, it is possible to use a method wherein apolymer (A) to be surface-crosslinked is dispersed in a large amount ofa low-boiling point organic solvent such as alcohol, ketone and ethercontaining water, and then a crosslinking agent is added to theresultant mixture, to thereby effect crosslinking (JP-A Sho. 57-44627);a method wherein a crosslinking agent is added to a polymer (A)containing water wherein the water content is adjusted to 10 to 40 wt. %to thereby effect crosslinking (JP-A Sho. 59-62665); a method wherein acrosslinking agent and water are absorbed into a polymer (A) in thepresence of an inorganic powder, and the resultant mixture is heatedwith stirring, so as to simultaneously effect crosslinking and removalof water (JP-A Sho. 60-163956); a method wherein 1 wt. part of a polymer(A) is dispersed into a large amount of a hydrophilic inactive solventhaving a boiling point of 100° C. or higher, in the presence of inactiveinorganic powder and 1.5 to 5.0 wt. parts of water, to thereby effectcrosslinking (JP-A Sho. 60-14745); a method wherein a polymer (A) istreated with a crosslinking agent and an aqueous solution containing anyof an alkylene oxide adduct of monohydric alcohol, a monovalent salt oforganic acid, and a lactam, to thereby effect reaction (JP-A Hei7-33818); etc.

(Polymer having carboxyl group)

Examples of an embodiment of the polymer (A) having calcium ionabsorption suitable for retaining water for a plant and also having apreferred water absorption magnification in ion-exchange water mayinclude, e.g., a polymer (A) having a carboxyl group bonded to thepolymer chain thereof wherein the polymer chain is crosslinked, and thecontent of an alkali metal salt of ammonium salt of the carboxyl groupis 0.3 to 7 mmol per 1 g of the polymer.

In the polymer (A), this carboxyl group may be non-neutralized, but maypreferably be the former (i.e., one containing alkali metal salt orammonium salt in an amount of 0.3-7 mmol per 1 g).

This “content of alkali metal salt or ammonium salt of carboxyl group”may preferably be 0.5-6.5 mmol (particularly, 1.0-6.0 mmol). The contentof the alkali metal salt of the carboxyl group may preferably bemeasured, e.g., by the following method.

(Method of measuring content of carboxyl group salt)

A water-retaining support is sufficiently washed with ion exchangewater, and then dried. 0.2 g of the dried water-retaining support isweighed in a platinum crucible, is subjected to ashing in an electricfurnace, and thereafter the support is dissolved in 5 ml of1N-hydrochloric acid. Then, distilled water is added to the resultantmixture so as to provide a total volume of 50 ml (constant volume), andthe positive ion concentration (D mM) therein is determined according toatomic absorption spectrometry. Since the concentration of the drywater-retaining support in the solution used herein for the atomicabsorption analysis is 0.2 g/50 ml=4g/1 L (liter) as described above,the content of carboxyl group salt in 1 g of the dried water-retainingsupport is calculated as D/4 (mmol).

At this time, in the case of the above positive ion measurement by theatomic absorption spectrometry, conditions similar to those in the caseof “calcium ion analysis” as described above can suitably be used.

The conventional hydrogel comprising a crosslinked product of an alkalimetal salt of polyacrylic acid has a water absorption magnificationwhich is markedly higher than that of a hydrogel comprising acrosslinked product of a nonionic hydrophilic polymer, and has been usedas a water-retaining support in the agricultural field because of such ahigh water absorption magnification. However, according to the presentinventor's experiments, in the hydrogel comprising the crosslinkedproduct of the alkali metal salt of polyacrylic acid which hasconventionally been developed as one to be used for agriculture, thecontent of the introduced dissociative ion groups is very high (e.g.,the amount of the introduced alkali metal salt of acrylic acid is about8 mmol or more per 1 g of the dry resin), whereby the hydrogel has atendency to adsorb heavy metal ions such as calcium ions which areessential for the growth of a plant, and it markedly inhibits the growthof the plant, as described hereinabove.

In contrast thereto, according to the present inventors' experiments, ithas been found that when 0.3 to 7 mmol of a dissociative ion group)e.g., alkali metal salt or ammonium salt of carboxyl group) isintroduced into a water-retaining support per 1 g of the dry support,the support shows a water-retaining effect (water absorptionmagnification in ion-exchange water of 10 to 1,000) which is sufficientfor growing a plant without causing deficiency of calcium ions in theplant.

Here, as the alkali metal salt, a sodium salt or a potassium salt ispreferred. When the content of the alkali metal salt of carboxyl groupis less than 0.3 mmol per 1 g of the dry water-retaining support, it isdifficult for the water-retaining support to have a water absorptionmagnification of 10 or more. On the other hand, when the content ofalkali metal salt of carboxyl group exceeds 7 mmol, the calcium ionabsorption is liable to exceed 100 mg or more per 1 g of the drywater-retaining support.

(Monomer)

The above polymer (A) may be obtained, e.g., by the ternarypolymerization of a monomer (I) having an alkali metal salt or ammoniumsalt of carboxyl group, a hydrophilic monomer (II), and a crosslinkingmonomer (III).

Herein, specific examples of the monomer (I) may include alkali metalsalts or ammonium salts of acrylic acid, methacrylic acid, maleic acid,itaconic acid, etc. These monomers may be either polymerized as a saltof monomer, or polymerized as a carboxylic acid monomer and thenconverted into a salt thereof by neutralization after thepolymerization. However, the content thereof may preferably be set to0.3 to 7 mmol per 1 g of the water-retaining support.

Specific examples of the hydrophilic monomer (II) may include acrylicacid, methacrylic acid, maleic acid, itaconic acid, acrylamide,methacrylamide, N-vinylacetamide, etc. When a monomer containing acarboxylic acid is used as the hydrophilic monomer (II), the resultanthydrogel has a tendency to have a low pH value. Accordingly, in thiscase, the alkali metal salt or ammonium salt content of the carboxylgroup may preferably be set to 1.0 to 6.0 mmol per 1 g.

In such a case, it is also possible to convert a portion of the monomercontaining the carboxylic acid into calcium salt so as to becopolymerized. According to the present inventors' investigation it hasbeen found that such a calcium salt-type monomer shows an effect ofdecreasing the calcium ion absorption of the water-retaining support, aneffect of avoiding a decrease in pH, and further an effect ofaccelerating the polymerization.

Specific examples of the crosslinking monomer (III) may includeN,N′-methylene bis(meth)acrylamide, N,N′-ethylene bis(meth)acrylamide,ethylene glycol di(meth)acrylate, and diethylene glycoldi(meth)acrylate, etc. The amount of the crosslinking monomer (III) tobe used may generally preferably in the range of 0.01 to 5 mol %, morepreferably in the range of 0.1 to 1 mol % with respect to all themonomers (while somewhat depending on the concentration for thepolymerization). When the amount of the monomer to be used is less than0.01 mol %, the strength of the water-retaining support tends to becomeinsufficient. On the other hand, when the amount of the monomer to beused exceeds 5 mol %, it becomes difficult for the water-retainingsupport to have a water absorption magnification of 100 or more.

It is also possible to obtain the polymer (A) by the saponification of acopolymer comprising vinyl acetate and maleic anhydride, a copolymercomprising vinyl acetate and acrylic acid (or salt thereof), etc. Thethus obtained polymer compound is a polyvinyl alcohol-type polymer. Whensuch a polymer is prepared so as to provide a content of alkali metalsalt or ammonium salt of the carboxyl group bonded to the polymer of 0.3to 7 mmol per 1 g of the dry weight, it is possible to obtain awater-retaining support according to the present invention having acalcium ion absorption of 0-100 mg per 1 g of the water-retainingsupport and having a water absorption magnification in ion-exchangewater of 10 to 1,000.

(Treatment with calcium ions)

The polymer (A) may also be obtained by treating a commerciallyavailable polyacrylate-type highly water-absorbing resin with a strongacid or calcium ions. In general, in the commercially availablepolyacrylate-type highly water-absorbing resin, at least 70% of thecarboxyl groups bonded to the polymer chain are in the state of alkalimetal salts, and the content thereof is at least about 8 mmol per 1 g ofthe resin. Therefore, the calcium ion absorption per 1 g of the resinbecomes 120 mg or more, and therefore it is inappropriate as thewater-retaining support for a plant.

In the present invention, a preferred embodiment is a polymer (A)containing a “polyvalent metal salt of carboxyl group”. As the ions ofthe polyvalent metal salt, there are exemplified, e.g., Ca²⁺, Mg²⁺,Al³⁺, Ba²⁺, Sr²⁺, B³⁺, Be²⁺, Fe²⁺, Fe³⁺, Mn²⁺, etc. Among these, Ca²⁺,Mg²⁺, Al³⁺, Ba²⁺, Sr²⁺, B³⁺, and Be²⁺ are preferred.

The polyvalent metal salt content may be 0.1-7 mmol, preferably 0.5-6.5mmol, more preferably 1.0-6.0 mmol, per 1 g of the dry weight of thepolymer (A). Such a content of the polyvalent metal salt of carboxylgroup may preferably be measured, e.g., by the following method.

(Method of measuring content of carboxyl group polyvalent metal salt)

A water-retaining support is sufficiently washed with ion exchangewater, and then dried. 0.2 g of the dried water-retaining support isweighed in a platinum crucible, is subjected to ashing in an electricfurnace, and thereafter the support is dissolved in 5 ml of 1N-hydrochloric acid. Then, distilled water is added to the resultantmixture so as to provide a total volume of 50 ml (constant volume), andthe calcium concentration (E mM) there is determined according to atomicabsorption spectrometry. The content of carboxyl group polyvalent metalsalt in 1 g of the dried water-retaining support is calculated asE×(valence number)/4 (mmol). When polyvalent metal ions are mixed, thevalence number obtained by subjecting the respective valence numbers ofthe polyvalent metal ions to “weighed-mean” treatment is used as thevalence number of the polyvalent metal ions (e.g., valence number=2 inthe case of Mg ions).

When a strong acid such as hydrochloric acid, nitric acid and sulfuricacid, or an aqueous calcium ion solution such as calcium chloridesolution and calcium nitrate solution is added to such a commerciallyavailable polyacrylate-type highly water-absorbing resin, the alkalimetal salt of carboxyl group in the highly water-absorbing resin issubstituted by the carboxylic acid or calcium salt of carboxyl group.Therefore, when the amount of the strong acid or calcium ions to beadded is appropriately set, the content of alkali metal salt of thecarboxyl group bonded to the polymer may be adjusted to 0.3 to 7 mmolper 1 g of the dry water-retaining support, to thereby provide awater-retaining support for plants, according to the present invention,having a calcium ion absorption of 0-100 mg per 1 g of the dry weightand having a water absorption magnification in ion-exchange water of 10to 1,000.

Here, when the carboxyl group is substituted by the carboxylic acid, theresultant hydrogel has a strong tendency to become acidic. Accordingly,particularly in this case, the content of alkali metal salt of carboxylgroup may preferably be adjusted to be 1.0 to 6.0 mmol per 1 g of thedry water-retaining support.

(Method of introducing chlorine ions)

As a method of causing the water-retaining support for plant accordingto the present invention to contain chlorine ions, it is possible toeffect the introduction by causing a polymer (A) to absorb an aqueoussolution containing chlorine ions. Further, in the case of synthesizingthe polymer (A) in water, it is more preferred to adopt a method ofincorporating chlorine ions in this aqueous solution. At this time, theamount of the addition of the chlorine ions is set to a value so thatthe amount thereof is 0.07-7 mmol per 1 g of dry weight of thewater-retaining support for plants to be provided.

The kinds of the counter positive ions with respect to the chlorine ionsare not particularly limited, but they may preferably be Na, K, Ca, andNH₄ ions. Among these, Na ions may particularly preferably be used.

Further, in a case where the polymer (A) already contains chlorine ionsin an amount of 7 mmol or more per 1 g of the dry weight thereof, it isalso possible to reduce the content of chlorine ions to a desired setvalue by washing the polymer (A) with water which contains no chlorineions (or, water having a low chlorine ion concentration).

Particularly, when a polyacrylic acid salt-type polymer (A) is used asthe water-retaining support for plant according to the presentinvention, it is preferred to utilize a method of forming a crosslinkedpolymer comprising acrylic acid and an acrylic acid alkali metal salt,and then adding thereto a polyvalent metal salt. Particularly, whencalcium chloride, or magnesium chloride, is used as the polyvalent metalsalt, it is possible to suppress the Ca absorption amount of thehydrogel, and to introduce chlorine ions into the hydrogel bysubstituting the alkali metal salt with Ca or Mg.

(Seed germination and germination activity test)

In order to evaluate the effect of a water-retaining support upon aplant, it is preferred to conduct a germination activity test for a seedby using, as a culture medium, the water-retaining support (hydrogel)which has absorbed agricultural water therein. For example, seeds ofwhite radish sprouts (e.g., those sold by Takii Shubyo K. K.) which mayeasily be subjected to a short-term germination activity test may beused as a seed material, and synthetic water having a typicalunderground water composition (Table 1) may be used as the agriculturalwater in the above-mentioned test.

TABLE 1 Composition Of Synthetic Water Component Concentration (mg/L)Ca(No₃)₂ · 4H₂O 272 MgSO₄ · 7H₂O 111 KCl  22 NaHCO₃ 126

(Respective components were dissolved in ion-exchange water at itspredetermined concentration, and the pH of the resultant mixture wereadjusted to 7 by using hydrochloric acid).

For example, the seed germination activity test may be performed in thefollowing manner. 16 ml of the above-mentioned synthetic water and 160mg (1 wt. %) of each kind of water-retaining support are introduced intoa test tube (having a diameter of 2.5 cm and a height of 15 cm), and theresultant mixture is fully stirred, and then the mixture is leftstanding for 30 minutes at 25° C., to thereby prepare a gel-like culturemedium comprising the water-retaining support which has absorbed theagricultural water therein. 5 grains of the above-mentioned seed ofwhite radish sprouts are uniformly put on the surface of the gel-likeculture medium in each of test tubes, and the test tube is capped with asilicone plug having a 6 mm-diameter hole filled with cotton. The thuscapped test tube is cultured for 4 days in a culture room (25° C.illumination intensity of 2000 Lux, 16h-daytime), and the resultantgermination activity is investigated.

In the above-mentioned germination activity test, for example, thelength of the above-ground portion measured as the average stem and leaflength from the base portion (branching point between the roots andstems) of the germinated individual to its leaf tip, while the length ofthe underground portion may be measured as the average root length fromthe base portion of the germinated individual to the tip of its mainroot.

In order to compare the respective germination activity test resultsmore precisely, it is preferred to use a standard hydrogel (the amountof calcium absorption thereof is less than 10 mg/g) as a control samplefor the respective germination activity tests, and to express the lengthof the above-ground portion and the length of the underground portionfor the respective hydrogels as relative values (%) with respect tothose for the standard hydrogel, as shown in Table 1.

(Method of using water-retaining support)

As the water-retaining support according to the present invention, thepolymer (A) may be used either singly or in combination with anotherplant-growing support (B) as desired.

In the present invention, the plant-growing support (B) is notparticularly restricted, but it is possible to use substances which havegenerally been used as those suitable for plant body-growing support. Assubstances suitable for plant body-growing, it is possible to usewater-insoluble solid-like substances such as powders of inorganicsubstances and/or organic substances, porous materials, pellet-likematerials, fibrous materials and foamed materials. However, in thepresent invention, various additives as described hereinafter (such aspigments, fertilizers, and anti-blocking agents) are not included inthis “plant body-growing support (B)”.

The examples of the inorganic substances may include, e.g., inorganicpowders (soil, sand, fly ash, diatomaceous earth, clay, talc, kaolin,bentonite, dolomite, calcium carbonate, alumina, etc.); inorganic fibers(rock wool, glass fiber, etc.); inorganic porous materials [filton(Kuntan, porous ceramic), vermiculite, pumice, volcanic ashes, zeolite,silas balloon, etc.]; inorganic foamed materials, (perlite, etc.), etc.

The examples of the organic substance may include, e.g., organic powders[crushed coconut shell, chaff, husk of peanut, husk of orange, woodshavings, wood powder, powder of dry coconut, synthetic resin powder(polyethylene powder, polypropylene powder, ethylene-vinyl acetatecopolymer powder]; organic fibers [natural fibers [cellulose-type fibers(cotton, sawdust, straw, etc.) and the like, grass peat, wool, etc.],artificial fibers (cellulose-type fibers such as rayon and acetate),synthetic fibers (polyamide, polyester, acrylic, etc.), pulps[mechanical pulp (ground pulp from logs, Asplund process ground pulp,etc.), chemical pulps (sulfite pulp, soda pulp, sulfate pulp, nitratepulp, chlorinated pulp, etc.), semi-chemical pulp, regenerated pulps(e.g., mechanically crushed or ground product from papers which haveonce been produced by forming pulp into papers, or regenerated pulpsfrom waste-papers as mechanically crushed or ground product fromwaste-papers, etc.)], other wasted materials (waste materials producedfrom paper diaper, etc.]; organic porous materials (activated carbonscoconut shell, etc.); organic foamed materials [cereals, foamed materialof synthetic resins or rubbers (polystyrene foamed material, polyvinylacetal-type sponge, rubber sponge, polyethylene foam, polypropylenefoam, urethane foam, etc.), etc.]; organic pellets [pellets of syntheticresins or rubbers, etc.], etc.

The above-mentioned plant body-growing support may be used singly or incombination of two or more species thereof, as desired. Among these, itis preferred to use inorganic porous materials, inorganic foamedmaterials, organic fibers, rubbers and synthetic resins. The density ofthe foamed material may preferably by 0.01-1 g/cm³, more preferably0.01-0.8 g/cm³, particularly 0.01-0.6 g/cm³.

(Organic foamed materials)

With respect to the synthetic resins and/or rubbers constituting theabove-mentioned organic foamed materials, it is possible to use thosewhich have been used generally.

More specifically, examples of the rubber may include, e.g.,styrene-butadiene rubbers (SBR), butadiene rubbers (BR), isoprenerubbers, butyl rubbers (IIR), ethylene-propylene rubbers,ethylene-propylene non-conjugated diene rubbers, polychloroprene rubbers(CR), nitrile rubbers, acrylonitrile-butadiene rubbers, in addition tothe usual natural rubbers (NR).

As the synthetic resin, thermoplastic resins or thermosetting resins maybe used.

(Thermoplastic resins)

As the thermoplastic resin, it is possible to use both of soft and hardresins. Examples thereof may include, e.g., ethylene-vinyl acetatecopolymers or saponified product thereof, ethylene-acrylic acid saltcopolymers, ethylene-acrylic acid ester copolymers, chlorosulfonatedpolyethylene, chlorinated polyethylene, urethane-type resins,styrene-type resins, vinyl chloride-type resins, olefin-type resins,polyester-type resins, polyamide-type resins, etc. Among these, it ispreferred to use those having a flexibility of a certain degree suchthat the volume thereof can be swollen by water absorption. When a hardresin is used, it is preferred to impart a flexibility to such a resinby using a suitable plasticizer.

Examples of the urethane-type resins may include, e.g., those producedby a method wherein a straight chain-type polyurethane obtained bybulk-polymerizing or solution-polymerizing a polyol, a diisocyanate, anda chain extender is formed into pellets, and then is extrusion molded orinjection molded; those produced by a method where a polyurethanesolution obtained from solution polymerization is shaped and thesolution is removed by evaporation; or those produced by a methodwherein such a solution is caused to contact a solidifying medium sothat it is solidified.

Examples of the styrene-type resins may include, e.g., styrene polymers,styrene-butadiene-styrene block copolymers, styrene-isoprene-styreneblock copolymers, styrene-ethylene-butylene-styrene block copolymers,styrene-ethylene-propylene-styrene block copolymers, etc.

Examples of the vinyl chloride-type resins may include, e.g.,high-polymerization degree vinyl chloride resins, partially crosslinkedvinyl chloride resins, nitrile rubbers (NBR), urethane resins or blendsof polyester resin, etc., and vinyl chloride resin, urethane-vinylchloride copolymers, nitrile rubber (NBR)-vinyl chloride copolymer, etc.

Examples of the olefin-type resins may include, e.g., polyethylene,polypropylene, mixtures of polyolefin with ethylene-propylene rubber,polymers comprising polyolefin grafted to ethylene-propylene rubber,etc.

Examples of the polyester-type resins may include, e.g., aromaticpolyester-polyether block copolymers, aromatic polyester-aliphaticpolyester block copolymers, etc.

Examples of the polyamide-type resins may include, e.g.,polyether-polyamide block copolymers, polyester-polyamide blockcopolymer, etc.

The molecular weight of these thermoplastic resins and rubbers are notparticularly limited, but they usually have a softening point of 30-300°C., preferably 40-200° C., particularly preferably 50-150° C. Thesematerials may be used singly or as a mixture of two or more speciesthereof as desired.

(Thermosetting resin)

Examples of the thermosetting resins may include, e.g., those offormalin-condensation resin-type, epoxy resin-type, urethane resin-type,etc.

Examples of the formalin-condensation resin-type may include, e.g., urearesins (reaction products from urea and formalin), melamine resins(reaction products from melamine and formalin), phenolic resins(reaction products from phenol and formalin), resorcinol resins(reaction products from resorcinol and formalin), etc.

Examples of the epoxy resin-type may include, e.g., products obtained bycombining a suitable hardening agent and an oligomer having reactiveepoxy group(s) at the end thereof, and a molecular weight of severalhundred to about 10,000, and hardening the former. More specificexamples thereof may include, e.g., reaction products (ratio of epoxygroups and each functional group is a molar ratio of 1:10 to 10;1)obtained from an epoxy resin (epoxy equivalent of 65-1000) such asglycidyl ether-type epoxy resins, glycidyl ester-type epoxy resins,glycidylamine-type epoxy resins, and alicyclic-type epoxy resins, and ahardening agent (such as polyamine, acid anhydride, polyisocyanate,polyol, and polymercaptan).

Examples of the urethane resin-type may include those obtained by amethod wherein a straight chain-type polyester, polyether or a polyesteramide as a base material is reacted with a polyisocyanate so as to forman NCO-terminated prepolymer (NCO percentage: 1-10%), and a chainextender is reacted with the prepolymer so as to form a polymer, andhardening the polymer by using heat or an appropriate crosslinking agent(prepolymer method); and those obtained by a method wherein a polyol, adiisocyanate, a chain extender, and a crosslinking agent are mixedsimultaneously, and are reacted so as to form a polyurethane (one-shotprocess) (isocyanate/(active hydrogen of polyol, etc.)≈0.8/1 to 10/1).Such a urethane resin may be molded or shaped by, e.g., a castingmethod, a kneading method.

(Molecular weight)

The number-average molecular weight of the above rubber andthermoplastic resin may usually be 1×10⁴ or more, preferably 2×10⁴ to100×10⁴. In addition, the number-average molecular weight of thethermosetting resin (before hardening) may usually be 10×10⁴ or less,preferably 5×10⁴ or less. The number-average molecular weight may bemeasured, e.g., by gel permeation chromatography (GPC method).

In addition, the size of the form of the support (B) is not particularlylimited, but the particle size (major axis) of the powder may usually be1-800 μm, preferably 5-200 μm, and the size of the porous material,fiber and foamed material may usually be 0.001-20 mm, preferably 0.01-10mm. The size of the pellet may usually be 0.1-50 mm, preferably 0.1-20mm.

(Ratio of quantity of polymer (A)/support (B))

The weight ratio between the polymer (A) and support (B) in the plantbody-growing support water-retaining material according to the presentinvention may be changed in various ways depending on the kind of thepolymer (A), the kind of the support (B), the optimum water content fora plant, but this ratio may usually be 0.1:99.9-80:20, preferably1:99-75:25, more preferably 5:95-70:30, particularly preferably10:90-65:35. Usually, when the ratio of the polymer (A) is 0.1 or more,enough water-retaining ability may be provided. It is desirable in viewof good formability and an economical point of view to use the materialat a ratio of 80 of the polymer (A) or less.

(Binder)

The plant body-growing water-retaining material according to the presentinvention comprises at least the above-mentioned polymer (A) and thesupport (B), but it may also comprises a binder (C) as desired.

Examples of the binder (C) may include those which have generally beenused, and may be either water-soluble or water-insoluble. When thepolymer (A) contains water, it usually has an adhesive property per se.However, it is possible to use the binder (C) as desired, in order toenhance the shaping effect of the polymer (A) and the support (B)depending on the state of water content, shape, specific gravity, etc.,of the polymer (A).

The form of the binder (C) is not particularly limited, but it maypreferably be one which is to be used at a state at which is has afluidity corresponding to a viscosity at 25° C. of 1000 Pa.S or less,more preferably 100 Pa.S or less. The binder (C) can be used in a statethereof such that it is dissolved or dispersed, e.g., in a solventand/or water.

Herein, the above-mentioned “viscosity” may preferably be measured,e.g., under the following conditions.

<Conditions for viscosity measurement>

Machine model: Rotary viscometer, mfd. by Tokyo Keiki Co., Ltd., tradename: BH-type Viscometer

Rotor: No. 1 to No. 7 (selected depending on the viscosity)

Number of revolutions of rotor: 2 rpm

The binder may appropriately be selected and used depending on themolding method to be used therefor. Examples of the binder (C) mayinclude, e.g., natural polymers, semi-synthetic polymers, syntheticresins and synthetic rubbers (however, the additive as describedhereinafter is not included in this binder (C)).

As the binder (C) which is water-soluble or water-dispersible and isusable in an aqueous system, it is possible to use, e.g., naturalpolymers on semi-synthetic polymers.

Example of the natural polymer may include, e.g., starch-like material(such as starch); animal proteins (such as gelatine, casein, andcollagen); animal proteins (such as soy bean protein, and wheatprotein); cellulose-type materials (such as wood cellulose); seaweedextracts (such as agar, and carrageenan); plant seed mucilages (such asguar gum, locust beam gum, tamarind seed gum); plant tree leaf mucilages(such as gum arabic, traganth gum); plant fruit mucilages (such aspectin); microbial mucilages (such as xanthan gum, pullulan, curdlan,dexetrin, gellane gum); plant underground stem mucilages (such askonnyaku mannan), etc.

Example of the semi-synthetic polymer may include cellulose derivatives(such as methyl cellulose, ethyl cellulose, hydroxy ethyl cellulose,ethyl hydroxy ethyl cellulose, carboxymethyl cellulose, methylhydroxypropyl cellulose); starch derivatives (such as soluble starch,carboxymethyl starch, methyl starch); and alginic acid derivatives (suchas alginic acid salts, alginic acid propylene glycol); etc. It is alsopossible to utilize the thermoplasticity of these substances, instead ofutilizing them in an aqueous system. The softening points of thesethermoplastic substances are the same as those described above in caseof the above-mentioned thermoplastic resin.

As the thermoplastic resin and substances which are soluble ordispersible in a solvent, it is possible to use rubbers and syntheticresins, etc. The examples of the rubbers and synthetic resins may be thesame as those as described above in the case of the support (B).

The amount of the binder (C) to be used as desired in the presentinvention may usually be 0-200 wt. %, preferably 0.5-150 wt. %,particularly 1-50 wt. %, in terms of the solid content thereof, withrespect to the total amount of the polymer (A) and support (B).

Production process)

As the process for producing the plant-growing water-retaining supportaccording to the present invention, e.g., the following methods may beused:

(i) a method wherein a mixture comprising the polymer (A), support (B)and optionally the binder (C) which has been mixed under stirring ispressure-molded into a pellet-like shape in a mold having appropriateshape and size;

(ii) a method wherein a mixture is pressure-molded, and is cut andcrushed into an appropriate size;

(iii) a method wherein the above-mentioned cut and crushed product isdusted or sprinkled with a polymer (A) and optionally a binder (C), andthen is again pressure-molded, and cut and crushed;

(iv) a method wherein a mixture used in the above method (iii) beforethe pressure molding is pressure-molded into a pellet-like shape in amold having appropriate shape and size;

(v) a method wherein a material is once pressure-molded into a productin the form of a sheet, rod, or block, and then is cut and crushed intoan appropriate size;

(vi) a method wherein a mixture is heat-molded into a product in theform of a sheet, rod, or block, and then is cut and crushed into anappropriate size;

(vii) a method wherein a mixture is heat-molded into a pellet-like shapein a mold having appropriate shape and size;

(viii) a method wherein a mixture is foamed into a product in the formof a sheet, rod, or block, and then is cut and crushed; etc.

In the above methods, it is also possible to further foam the materials,as desired. In addition, it is possible to thicken the hydrogel byadding water in an amount of 1-50% based on the total amount of thepolymer (A), support (B) and binder (C) and mixing these materials atthe time of mixing of the polymer (A), support (B) and optionally thebinder (C).

Among the above-mentioned methods, the methods (ii), (vi), (vii), and(viii) may preferably be used.

The shape of the water-retaining material to be obtained by theabove-mentioned method according to the present invention is notparticularly limited, but it is usually preferred to obtain the materialin the form of a molded product.

Examples of the molded product may include those selected from the groupof: pressure-molded pellet-like products; cut or crushed products fromthe pressure-molded sheet-like material, rod-like material, orblock-like material; cut or crushed products from the heat-moldedsheet-like material, rod-like material, or block-like material; and cutor crushed products from the foamed sheet-like material, rod-likematerial, or block-like material. Among these, the products maypreferably be cut or crushed products from the pressure-molded orheat-molded sheet-like material, rod-like material, or block-likematerial; pressure-molded products of pellet-like molded product; or cutor crushed products from the foamed sheet-like material, rod-likematerial, or block-like material.

(Foamed product)

The plant body-growing water-retaining material according to the presentinvention can be formed into a foamed product as desired. In the case ofobtaining such a foamed product, when the above support (B) isthermoplastic resin and rubber, it can be produced by compounding afoaming agent (and foaming promoter or foaming inhibitor as desired)into the polymer (A), support (B), and then heat-foaming the resultantmixture. Examples of the usable foaming agent may include: diazoaminoderivatives, azonitrile, azodicarboxylic acid derivatives,dinitropentamethylene tetramine (DPT), benzene monohydrazol,oxybisbenzene sulphonyl hydrazide (OBBH), ammonium carbonate, ammoniumbicarbonate, propane, petroleum ether, etc. These agent may preferablybe used in an amount in the range of 1-80 mass part with respect to 100mass part of the support (B), while it is somewhat different dependingon the expansion ratio or usage for the foamed product. In addition, itis possible to mix a plasticizer, a stabilizer, a lubricant, a filler, acoloring agent, a flame retardant, an antistatic agent, or amildewproofing agent at the time of preparing a mixture of the polymer(A) and support (B) as desired. In addition, when a rubber is used asthe support (B), it is possible to mix a rubber reinforcing agent, atackifier, a processing aid, an antioxidant, an infrared ray-absorbingagent, an aging (ozone) resistor, an agent for rubber such asvulcanizing agent, vulcanizing promoter, vulcanizing aid or activator,etc.

The foaming may be effected by usual one-step foaming or two-stepfoaming. The density of the resultant foamed product is not particularlylimited.

When the above support (B) is a thermosetting resin, e.g., it issufficient that the polymer (A) and the support (B) are mixed in advanceat the time of producing a usual urethane foam so as to foam a urethaneresin containing the polymer (A). The procedure for such a process maybe the same as that for the production of a usual urethane foam. A usualpolyurethane foam may be provided by a one-shot method wherein apolyisocyanate and a polyhydroxyl compound are reacted in one stage inthe presence of a foaming agent and an appropriate aid; or by a totalprepolymer method wherein a prepolymer which has been obtained byreacting an excess amount of polyisocyanate and a polyhydroxyl compound,is reacted with water in the presence of an appropriate aid; or by asemi-prepolymer method wherein a prepolymer which has been obtained byreacting an excess amount of polyisocyanate and a polyhydroxyl compound,is reacted with an additional amount of a polyhydroxyl compound in thepresence of a foaming agent and another appropriate aid. Herein,examples of the foaming agent may include a reactive foaming agent suchas water, and another non-reactive foaming agent such as low-boilingpoint halogenated hydrocarbon. The “other aid” means a catalyst,foaming-regulating agent (bubble stabilizer), a coloring agent, etc.

The above-mentioned mixing device may be any device which is capable ofmixing a mixture uniformly. Examples of such a mixing device mayinclude, e.g., a Henschel mixer, a ribbon blender, a planetary mixer, atumbler, a universal mixer, etc. In addition, examples of the device forkneading a mixture may include, e.g., devices which is capable ofeffecting kneading operation under heating and shearing force, such astwin-screw extruder, single-screw extruder, co-kneader, Banbury mixer,kneader, and open-roll device.

(Other molding methods)

In the case of pressure-molding method, it is possible to use, e.g., adry-type pressure-molding method, a direct powder pressure-moldingmethod, a wet-type pressure-molding method, etc. The pressure-moldingmay be effected by using a roll-type pressure-molding machine (such asbriquette machine), a piston-type pressure-molding machine, a screw-typepressure-molding machine, an perforated plate-extruding type moldingmachine (such as disk-pelletting machine), etc. Among thesepressure-molding machines, it is preferred to use a roll-typepressure-molding machine and/or a perforated plate-extruding typemolding machine. In addition, the pressurization at the time of thepressure-molding may usually be effected at normal temperature, but mayalso be effected under heating (for example, at 30-300° C.). It ispossible to appropriately select the pressure at the time of thepressure-molding in accordance with the kind, size (particle size),property of the base material, etc., but it is usually 1-3000 kg/ch²,preferably 10-2000 kg/ch². The shape of the resultant pressure-moldedproduct is arbitrary, and may be, e.g., various kinds of shape such assheet-like, spherical, cylinder-type plate-type, mass-type, rectangularparallelopiped-type, cone-type, pyramid-type, rod-type, etc. The size ofthese product may be, e.g., a thickness of 0.1-30 mm in the case ofsheet-type, a maximum diameter of 0.1-30 mm in the case of from thespherical type to the rod-type. The size of the cut product may bearbitrary. The size of the crushed product may usually be 0.001-20 mm,preferably 0.01-10 mm. The cutting may be effected by using a knownmethod, such as those using a cutter, a pelletizer, etc. The crushingmay be effected by using a known method, such as those using animpact-type crusher (such as pin mil, cutter mil, skillel mill, ACMpulverizer, centrifugal crusher), or an air-type crusher (such as ajet-mill).

In the case of a warming and/or a dry-molding method, it is possible toadopt various methods such as extrusion molding, press molding, acombination of extrusion molding and press molding, and centrifugalmolding, without particular limitation. In the case of the extrusionmolding as a representative example, a mixture according to the presentinvention is extrusion-molded into a desired shape, by means of ascrew-type vacuum extrusion molding machine, a screw-type extrusionmolding machine, a plunger-type molding machine, etc., through a diemounted at the tip of the machine, and the product is cut or crushedinto a desired length and size by using a cutter or crusher. Theextrusion-molded mixture is then heated and/or dried, to thereby providean intended molded product. The above drying method can be effected by aknown method, e.g., by using gas transmission drying (such as banddrying) and ventilation drying (such as air circulation drying), contactdrying (drying using a drum-type dryer, etc.) or vacuum orreduced-pressure drying, etc. In addition, the temperature at the timewarming and/or dry-molding can appropriately be selected in accordancewith the kind, size (particle size), property, etc., of the basematerial, but may usually be 30-300° C., preferably 50-200° c. In theabove procedure, drying may usually be effected at atmospheric pressure,but it is also possible to effect the drying under reduced pressure(750-5 mm Hg). The resultant shape of the heat- and/or dry-moldedproduct may be the same as those in the case of the pressure-molding.The water content of dried product may be 10% or less, preferably 7% orless.

(Other materials)

Further, it is possible use an agent such as fertilizer, agriculturalchemicals, insecticide, antibacterial agent, deodorant, flavoring agent,mildewproofing agent, antiseptic, anti-blocking agent, surfactant, etc.,in combination with the plant body-growing water-retaining materialaccording to the present invention as desired. It is sufficient thatsuch an agent is present in the plant body-growing water-retainingmaterial according to the present invention, and it is possible to addthe agent to the plant body-growing support and/or hydrogel-formingpolymer in advance, or to add the agent before or after the moldingprocess for the support or polymer.

The plant body-growing water-retaining material may be colored or notcolored, but may preferably be colored by using a pigment and/or dye inview of the visual effect thereof.

(Method of using water-retaining material)

With respect to the method of using the plant body-growingwater-retaining material, it is possible to use such a material singlyas a planter material, or in a method wherein the water-retainingmaterial is mixed with a cultivating bed material such as soil, a methodwherein the water-retaining material is charged into a specific sitewhich is remote from a plant, or a method wherein the water-retainingmaterial is buried in a cultivating bed material at an appropriate depththereof so as to form a layer of the water-retaining material. Ingeneral, the water-retaining material may generally be charged into aportion in the periphery of the seeding site, root-system developingportion, and soil surface layer portion. That is, the plant body-growingwater-retaining material according to the present invention may becharged into any portion of soil, as long as a water-retaining layer oran water-retaining mass is formed by using the water-retaining materialso that the water content retained by the water-retaining material iseffectively utilized by a plant to be cultivated. Further, it is alsopossible to use the plant body-growing water-retaining materialaccording to the present invention by incorporating such awater-retaining material into another material such as vegetation zone,vegetation mat, vegetation bag, and vegetation plate.

The plant body-growing water-retaining material has a property such thatis absorbs water or an aqueous solution (such as aqueous liquid whereina fertilizer ingredient is dissolved in water) so that it is swollen tohave a mass thereof which is preferably 5-200 times, more preferably10-100 times the mass of the plant body-growing water-retainingmaterial.

The “dispersion liquid” to be retained in a crosslinked or networkstructure is not particularly limited, as long as it is a liquidcomprising water as a main or major component. More specifically, thedispersion liquid may, for example, be either of water per se, anaqueous solution (e.g., aqueous liquid wherein a water-solublefertilizer ingredient, etc., is dissolved in water), and/or awater-containing liquid (e.g., a mixture liquid of water and amonohydric or polyhydric alcohol).

Hereinbelow, the present invention will be described in more detail withreference to Examples.

EXAMPLE 1

Into a one-liter beaker, 230 g of acrylic acid, 133 g of 48%-aqueoussodium hydroxide solution, 1.0 g of pentaerythritol triallyl ether, and636 g of water were added, and the resultant mixture was cooled to 10°C. The resultant solution was added to an adiabatic polymerizationvessel, was bubbled with nitrogen so as to adjust the dissolved oxygenof the solution to 0.1 ppm (measured by Dissolved Oxygen Meter O220PB(trade name), mfd. by Orient electric Company), and then 0.023 g of35%-aqueous hydrogen peroxide solution, 0.00575 g of L-ascorbic acid and0.23 g of potassium persulfate were added thereto. About 30 minute afterthe addition, polymerization reaction was initiated, and about 2 hourslater, the temperature reached the highest value of 72° C. Further, thereaction mixture was matured at this temperature for five hours, tothereby complete the polymerization.

The resultant polymer had a water-containing gel state. This polymer wasstirred by a kneader (trade name: BENCH KNEADER PNV-1, mfd. by IrieShokai; number of revolutions 70 rpm) for about 2 hours to thereby shredthe polymer. Further, 35.5 g of 50% aqueous calcium chloride solutionwas compounded therewith, and the mixture was stirred with a kneader forabout 2 hours to be mixed. The resultant product was subsequently heatedand dried at 110° C., and was crushed to thereby obtain awater-absorbing resin (hydrogel-forming polymer) having an averageparticle size of 450 micron (measured by means of Micro-track FRAParticle Size Analyzer (trade name), mfd. by Nikkiso Co.).

EXAMPLE 2

A water-absorbing resin was prepared in the same manner as in Example 1,except that 71 g of 50%-aqueous calcium chloride solution was usedinstead of 35.5 g of the 50%-aqueous calcium chloride solution at thetime of adding the inorganic salt solution to the kneader used inExample 1.

EXAMPLE 3

A water-absorbing resin was prepared in the same manner as in Example 1,except that 106.5 g of 50%-aqueous calcium chloride solution was usedinstead of 35.5 g of the 50%-aqueous calcium chloride solution at thetime of adding the inorganic salt solution to the kneader used inExample 1.

COMPARATIVE EXAMPLE 1

A water-absorbing resin was prepared in the same manner as in Example 1,except that 23.9 g of calcium hydroxide was used instead of 35.5 g ofthe 50%-aqueous calcium chloride solution at the time of adding theinorganic salt solution to the kneader used in Example 1.

COMPARATIVE EXAMPLE 2

Into a one-liter beaker, 230 g of acrylic acid, 186.7 g of 48%-aqueoussodium hydroxide solution, 1.0 g of pentaerythritol triallyl ether, and582.3 g of water were added, and the resultant mixture was cooled to 10°C.

The resultant solution was added to an adiabatic polymerization vessel,was bubbled with nitrogen so as to adjust the dissolved oxygen of thesolution to 0.1 ppm, and then 0.023 g of 35%-aqueous hydrogen peroxidesolution, 0.00575 g of L-ascorbic acid and 0.23 g of potassiumpersulfate were added thereto. About 30 minutes after the addition, apolymerization reaction was initiated, and about 2 hours later, thetemperature reached the highest value of 72° C. Further, the reactionmixture was matured at this temperature for five hours, to therebycomplete the polymerization.

The resultant polymer had a water-containing gel state. This polymer wasstirred by a kneader for about 2 hours to thereby shred the polymer.Further, 23.9 g of calcium hydroxide was compounded therewith, and themixture was stirred with a kneader for about 2 hours to be mixed. Theresultant product was substantially heated and dried at 110° C., and wascrushed to thereby obtain a water-absorbing resin having an averageparticle size of 450 micron.

COMPARATIVE EXAMPLE 3

A water-absorbing resin was prepared in the same manner as in Example 1,except that 104.9 g of 50%-aqueous calcium nitrate solution was usedinstead of 35.5 g of the 50%-aqueous calcium chloride solution at thetime of adding the inorganic salt solution to the kneader used inExample 1.

COMPARATIVE EXAMPLE 4

A water-absorbing resin was prepared in the same manner as inComparative Example 2, except that 104.9 g of 50%-aqueous calciumnitrate solution was used instead of 23.9 g of calcium hydroxide at thetime of adding the inorganic salt solution to the kneader used inExample 1.

EXAMPLE 4

10 g of commercially available polyacrylic acid-type hydrogel (tradename: Sanfresh ST-500D, mfd. by Sanyo Chemical Industries, Ltd.) wasswollen with 4 L of distilled water, and then 1 L of CaCl₂ solution (Cacontent 1 g, concentration 0.28%) was added thereto, and the resultantmixture was sufficiently stirred. The thus obtained product was leftstanding for about 2 hours while being occasionally stirred, theresultant gel was filtered by using a mesh (fineness of the mesh: nylonmesh filtration cloth, 250 mesh, mfd. by Asaka Roshi Company, trade nameN-No250HD), and the gel was dried in a dryer (120° C.) for one hour.After the drying, the resultant product was crushed by a mortar into gelpowder.

COMPARATIVE EXAMPLE 5

10 g of Sanfresh ST-500D used in Example 4 was swollen with 4 L ofdistilled water, and then 1 L of Ca(NO₃)₂ solution (Ca content 1 g) wasadded thereto, and the resultant mixture was sufficiently stirred. Thethus obtained product was left standing for about 2 hours while beingoccasionally stirred, the resultant gel was filtered by using a mesh,and the gel was dried in a dryer (120° C.) for one hour. After thedrying, the resultant product was crushed by a mortar into gel powder.

<Germination activity test using white radish sprouts>

Germination activity tests were conducted with respect to thewater-absorbing resins obtained in the above Examples 1-4 andComparative Examples 1-5 by using white radish sprouts in the manner asdescribed hereinabove. The thus obtained results are shown in thefollowing Table 2.

TABLE 2 Properties of respective samples and results of germinationactivity tests using white radish sprout Calcium ion Chlorine ionWater-absorbing Relative values of germination activities (%) absorption(mg/g) content (mmol/g) magnification (g/g) (above-groundportion/underground portion) Ex.-1 85.4 1.6 309 108/90  Ex.-2 62.8 1.9210 115/93  Ex.-3 42.0 2.5 105 108/91  Ex.-4 62.9 0.6 244 106/129 Comp.Ex.-1 100.0 0 270 73/50 Comp. Ex.-2 85.8 0 430 47/46 Comp. Ex.-3 47.4 0200 82/65 Comp. Ex.-4 39.8 0 210 61/54 Comp. Ex.-5 46.1 0 155 91/64Comp. Ex.-6 Sanfresh 164 0 302 43/7  Acryhope 150 0 196 38/14 Dia-Wet140 0 172 30/11 Sumicagel 110 0 326 14/8  Comp. Ex.-7 110 1.0 300 20/12Comp. Ex.-8 63.0 8.1  75 90/34

COMPARATIVE EXAMPLE 6

With respect to four kinds of commercially available polyacrylicacid-type hydrogels (trade name: San-Fresh, mfd. by Sanyo ChemicalIndustries, Ltd.; Acryhope, mfd. by Nippon Shokubai K.K.; trade name:Diawet, mfd. by Mitsubishi Chemical K.K.; and trade name: Sumicagel,mfd. by Sumitomo Chemical K.K.), the calcium ion absorption, chlorineion content, and water absorption magnification were measured, andgermination activity tests were conducted by using white radish sprouts.The thus obtained results are also shown in Table 1.

COMPARATIVE EXAMPLE 7

10 g Sumicagel used in Comparative Example 6 was added to about 100 mlof a saline solution having a sodium chloride concentration of about 0.6w %, and stirred sufficiently. When the gel was swollen, and no releasedwater was observed, the product was dried (120° C., for three hours),and then crushed by using a mortar to thereby obtain gel powder.

COMPARATIVE EXAMPLE 8

10 g the hydrogel prepared in Example 4 was added to about 300 ml of asaline solution having a sodium chloride concentration of about 3.0 w %,and stirred sufficiently. When the gel was swollen, the product wasdried (120° C., for 5 hours), and then crushed by using a mortar tothereby obtain gel powder.

EXAMPLE 5

“Water-absorbing resin A” (as shown in Table 2) having a calcium ionabsorption of 85.4 (mg/g), a chlorine ion content of 1.6 (mmol/g), awater absorption magnification of 309 (g/g) was obtained in the samemanner as in Example 1.

Silica sand “Natural Silica Sand No. 4” (particle size 20-65 mesh, mfd.by Tsuchiya Kaolin Co.) and the above water-absorbing resin A were mixedwith each other in a weight ratio of 85:15 by using a Henschel mixer ata number of revolutions of 100 rpm, for 15 minutes. Further, theresultant mixture was pressed at 2,000 kg/cm (line pressure) at roomtemperature, by means of a briquette machine (mfd. by Shinto Kogyo Co.)to thereby obtain a plant body-growing water-retaining material in theform of pellets of about 4 mm in size.

EXAMPLE 6

A plant body-growing water-retaining material was prepared in the samemanner as in Example 5, except that “Natural Silica Sand No. 4”,“Water-absorbing resin A”, and “Ruckstar CB-2” (styrene butadienerubber, mfd. by Dainippon Ink Co. were used in a mixing weight ratio of85:15:1, instead of the “Natural Silica Sand No. 4” and “Water-absorbingresin A” in a mixing weight ratio of 85:15 as used in Example 5.

EXAMPLE 7

85 parts of “Natural Silica Sand No. 4”, 15 parts of the Water-absorbingresin A prepared in Example 1, and 20 parts of “crushed pulp” were mixedwith each other in their powder state by using a Henschel mixer at anumber of revolutions of 100 rpm, for 15 minutes. Further, the resultantmixture was uniformly mixed by using a Henschel mixer at a number ofrevolutions of 100 rpm, for 3 minutes while the mixture was sprayed with30 parts of water. Then, the resultant mixture was charged into achopper (trade name: Disk Pelleter, mfd. by Fuji Powdal Co.) so that itwas extruded into a rod-shaped product (diameter of 4 mm). Thisrod-shaped product was cut into 5 mm-long pieces, and thereafter chargedinto a drier (trade name: Model SPHH-200 Safety Oven, mfd. byTabai-Espek Co.) so as to be dried at 80° C. for two hours, to therebyobtain a plant body-growing water-retaining material in the form ofpellets.

COMPARATIVE EXAMPLE 9

A plant body-growing water-retaining material was prepared in the samemanner as in Example 5, except that “Sanfresh ST-500D” was used insteadof the “Water-absorbing resin A” used in Example 5.

Sanfresh ST-500D: polyacrylic acid-type hydrogel, average particle size450 micron, calcium ion absorption 164 (mg/g), chlorine ion content 0(mmol/g), water absorption magnification of 302 (g/g), mfd. by SanyoChemical Industries Co.

[Degree of plant growth (1)]

5 kg of sandy soil (e.g., river sand) was charged into a plastic planterhaving sizes of 30 cm×20 cm 20×cm.

Soil which had been obtained by adding 1.3 kg of a plant body-growingwater-retaining material (each of those of Example 5-7, and ComparativeExample 9), and 0.5 kg of a chemical fertilizer (nitrogen:phosphoricacid:potassium=1:1:1) to 8.7 kg of sand soil, and sufficiently mixingwith each other, was charged into the above planter so as to form alayer, and the planter was sufficiently watered. Thereafter, the ease ofmixing (i.e., uniformity, prevention of “aggregates” formation) of theplant body-growing water-retaining material into the soil was observedwith the naked eye, and cucumber, Japanese radish, and rice plants wererespectively seeded into the planters. 50 g of ion exchange water wasirrigated every four days, and the states of the growth (mean values ofrespective 12 stocks) of the respective plants after 14 days wereobserved.

[Ease of mixing of plant body-growing water-retaining material intosoil]

The ease of mixing of the plant body-growing water-retaining materialinto the soil was observed with the naked eye. The evaluation wasconducted according to the following evaluation standards.

◯: uniform mixing was observed.

Δ: Slight non-uniformity was observed (i.e., gel blocking was notformed, but slight non-uniformity was recognized by naked-eyeobservation).

x: Gel blocking was formed, and non-uniform mixing was observed.

Degree of plant growth (1) tests were conducted by using the plantbody-growing water-retaining materials obtained in the above Examples5-7 and Comparative Example 9 in the same manner as described above. Thethus obtained results are shown in the following Table 3.

TABLE 3 Degree of growth of above- Easiness of mixing of ground portion(cm) plant-growing water- White retaining material Cucumber radish Riceinto soil Ex. 5 12.0 9.1 12.1 ◯ Ex. 6 11.8 9.0 12.5 ◯ Ex. 7 11.5 8.010.2 ◯ Comp. Ex. 9  2.0 1.8  1.3 x

EXAMPLE 8

“Perlite No. 3” (obsidian, mfd. by Nihon Cement Co.), “Water-absorbingresin A prepared in Example 5”, and “Powder Resin EVA5015M”(thermoplastic EVA resin, mfd. by Tokyo Ink Mfg. Co.) were mixed witheach other in a weight ratio of 10:1:5. 200 g of the resultant mixturewas charged into a pot having a capacity 250 cc, the surface of themixture was flattened. Further, the mixture was heated at 140° C. for 30minutes, and then was cooled to room temperature, to thereby prepare aplant body-growing water-retaining material having a shape-retainingproperty.

EXAMPLE 9

“Perlite No. 3” (obsidian, mfd. by Nihon Cement Co.), “Water-absorbingresin A prepared in Example 5”, and “Elastolan ET1040” (thermoplasticurethane resin, mfd. by Takeda Birdische Urethane Co.) were mixed witheach other in a weight ratio of 10:1:5 by means of a mixer (trade name:Omni Mixer Model OM-5, mfd. by Chiyoda Technical Industry Corporation)at a number of revolutions of 100 rpm for ten minutes. 200 g of theresultant mixture was charged into a pot-type container (material:pottery) having a capacity 250 cc, the surface of the mixture wasflattened by using a spatula. Further, the mixture was heated at 140° C.for 30 minutes so as to soften the above thermoplastic urethane resin,and then was cooled to room temperature, to thereby prepare a plantbody-growing water-retaining material having a shape-retaining property.

EXAMPLE 10

“Water-absorbing resin B” (as shown in Table 2) having a calcium ionabsorption of 62.9 (mg/g), a chlorine ion content of 0.6 (mmol/g), awater absorption magnification of 244 (g/g) was obtained in the samemanner as in Example 4. Further, a plant body-growing water-retainingmaterial having a shape-retaining property was prepared in the samemanner as in Example 9 except that the above “Water-absorbing resin B”was used instead of the “Water-absorbing resin A” prepared in Example 5”as used in Example 9.

EXAMPLE 11

“Perlite No. 3” (obsidian, mfd. by Nihon Cement Co.), “Water-absorbingresin A prepared in Example 5”, and “ES-Fiber”(polyethylene-polypropylene composite fiber, average fiber length 500micron, fiber diameter 5 micron, Chisso Co.) were mixed with each otherin a weight ratio of 10:1:5 in the same manner as in Example 10. 200 gof the resultant mixture was charged into a pot having a capacity 250cc, the surface of the mixture was flattened. Further, the mixture washeated at 140° C. for 30 minutes so as to soften the above ES-Fiber, andthen was cooled to room temperature, to thereby prepare a plantbody-growing water-retaining material having a shape-retaining property.

EXAMPLE 12

“Perlite No. 3” (obsidian, mfd. by Nihon Cement Co.), “Water-absorbingresin A prepared in Example 5”, and “Sanprene SEL No. 23”(NCO-terminated urethane resin, mfd. by Sanyo Chemical Industries, Ltd.,liquid state) were mixed with each other in a weight ratio of 10:1:5 inthe same manner as in Example 10. 200 g of the resultant mixture wascharged into a pot having a capacity 250 cc, the surface of the mixturewas flattened. Further, the mixture was left standing at roomtemperature so as to harden the above NCO-terminated urethane resin, tothereby prepare a plant body-growing water-retaining material having ashape-retaining property.

COMPARATIVE EXAMPLE 10

A plant body-growing water-retaining material was prepared in the samemanner as in Example 8, except that “Sanfresh ST-500D” was used insteadof the “Water-absorbing resin A” prepared in Example 5.

COMPARATIVE EXAMPLE 11

A plant body-growing water-retaining material was prepared in the samemanner as in Example 11, except that “Sanfresh ST-500D” was used insteadof the “Water-absorbing resin A” prepared in Example 5.

[Degree of plant growth (2)]

Each of the plant body-growing water-retaining materials having ashape-retaining property (each of those of Examples 8-12, andComparative Example 10 and 11) was immersed in a fertilizer solution(Hyponex 20-20-20, 1 g/L, mfd. by Hyponex Japan Co.) so that thematerial fully absorbed water. The upper surface of each sample wasstrongly pushed by a finger-tip so as to form concavities therein(diameter about 5 mm×depth 15 mm). Seeds (cucumber, Japanese radish, andrice; one seed per one concavity) were placed into the thus formedconcavities, and the water-absorbed water-retaining material around thecircumference of the concavities was broken down so as to cover eachseed. 50 g of exchange water was irrigated every four days, and thestates of the growth (mean values of respective 6 stocks) of therespective plants after 14 days were observed.

Degree of plant growth (2) tests were conducted by using the plantbody-growing water-retaining materials obtained in the above Examples8-12 and Comparative Example 10 and 11 in the same manner as describedabove. The thus obtained results are shown in the following Table 4.

TABLE 4 Degree of growth of above-ground portion (cm) Cucumber Whiteradish Rice Ex. 8 10.3 8.0 11.6 EX. 9 10.5 8.6 12.0 Ex. 10 11.0 9.0 12.2Ex. 11 10.0 8.0 10.0 Ex. 12 10.6 7.6 10.0 Comp. Ex. 10  0.8 0.2  1.7Comp. Ex. 11  0.5 0.2  1.3

EXAMPLE 13

100 parts of the above-mentioned “Water-absorbing resin A prepared inExample 5” was added to 100 parts of a thermoplastic urethane resin(trade name: Elastolan ET1040, mfd. by Takeda Birdische Urethane Co.),and the mixture was kneaded at 150° C. for 10 minutes by using anopen-roll, and was press-molded at 180° C. by means of a machine (tradename: Shind-type SF type Oil Pressure Press, mfd. by Sindo Kyogyo Co.)to thereby obtain a sheet having a thickness of 1.5 mm. This sheet wascut by means of a pelletizer (trade name: Pelletizer SGG-220, mfg. byHORAI K.K.) to thereby prepare a plant body-growing water-retainingmaterial in the form of 1.5 mm rectangular pellets.

EXAMPLE 14

A plant body-growing water-retaining material was prepared in the samemanner as in Example 13, except that a “styrene-type thermoplasticresin” (trade name: Taftec H1052, mfd. by Asahi Kasei Kogyo Co.) wasused instead of the “thermoplastic urethane resin” as used in Example13.

EXAMPLE 15

A plant body-growing water-retaining material was prepared in the samemanner as in Example 13, except that a “polyolefin-type thermoplasticresin” (trade name: TPE3570, mfd. by Sumitomo Chemical Co.) was usedinstead of the “thermoplastic urethane resin” as used in Example 13.

EXAMPLE 16

A plant body-growing water-retaining material was prepared in the samemanner as in Example 13, except that a “ethylene-vinyl acetatecopolymer” (trade name: Evatate R5011, mfd. by Sumitomo Chemical Co.)was used instead of the “thermoplastic urethane resin” as used inExample 13.

EXAMPLE 17

The water-absorbing resin A of Example 5, styrene-butadiene rubber (SBR,trade name: Exxonbutyl 268, mfd. by Exxon Chemical Co.) and a foamingagent dinitropentamethylene tetramine) were used, and these materialsaccording to the following mixing ratio were kneaded for 10 minutes bymeans of an open-roll machine, to thereby prepare a sheet having athickness of 3 mm. The thus obtained sheet was charged into a metal moldand was pressurized by means of a press at 145° C. for 20 minutes, themetal mold was removed and the product was cooled, to thereby prepare afoamed product (density 0.2 g/ml). Thereafter, the resultant foamedproduct was cut to thereby prepare 1.5 mm-rectangular chips.

SBR 50 parts

Water-absorbing resin A 50 parts

Zinc oxide 2.5 parts

Powder sulfur 1.0 part

Promoter (DM; dibenzothiazyl disulfide) 1.0 part

stearic acid 0.5 part

Paraffin 1.0 parts

Foaming agent (DPT) 1.5 parts

COMPARATIVE EXAMPLE 12

A plant body-growing water-retaining material was prepared in the samemanner as in Example 13, except that “San-Fresh ST-500MPS” was usedinstead of the “Water-absorbing resin A” used in Example 13.

Sanfresh ST-500MPS: polyacrylic acid-type hydrogel, average particlesize 35 micron, calcium ion absorption 164 (mg/g), chlorine ion content0 (mmol/g), water absorption magnification of 302 (g/g), mfd. by SanyoChemical Industries Co.

[Degree of plant growth (3)]

Each of the plant body-growing water-retaining materials having ashape-retaining property (each of those of Examples 13-17, andComparative Example 12) was immersed in a fertilizer solution (Hyponex20-20-20, 1 g/L, mfd. by Hyponex Japan Co.) so that the material fullyabsorbed water, and thereafter, charged into a flowerpot (material:pottery) having a capacity of 1L, and the surface of the material wasleveled. The upper surface of each of the thus obtained samples wasstrongly pushed by a finger-tip so as to form concavities therein. Seeds(cucumber, Japanese radish, and rice; one seed per one concavity) wereplaced into the thus formed concavities, and the water-absorbedwater-retaining material around the circumference of the concavities wasbroken down so as to cover each seed. 50 g of exchange water wasirrigated every four days, and the states of the growth (mean values ofrespective 6 stocks) of the respective plants after 14 days wereobserved.

The water absorption magnification and degree of plant growth (2) testswere conducted by using the plant body-growing water-retaining materialsobtained in the above Examples 13-17 and Comparative Example 12 in thesame manner as described above. The thus obtained results are shown inthe following Table 5.

TABLE 5 Water Degree of growth of above- absorption ground portion (cm)magnification White (g/g) Cucumber radish Rice Ex. 13 35 10.6 7.6 10.0Ex. 14 20 10.0 8.0 10.0 Ex. 15 23 10.3 8.0 11.6 Ex. 16 40 10.5 8.6 12.0Ex. 17 60 11.0 9.0 12.2 Comp. Ex. 12 32  0.8 0.2  1.7

Industrial Applicability

When the water-retaining support for plant according to the presentinvention is used, since the water-retaining support absorbs thereinonly a small amount of calcium ions and has a suitable chlorine ioncontent, a plant does not suffer from calcium ion deficiency. Inaddition, since the water absorption magnification of such a support issufficiently large, the support can supply sufficient water to a plant.

Since the plant body-growing water-retaining material according to thepresent invention does not obstruct the growth of a plant, and isexcellent in water-absorbing ability, such a material can supplysufficient water to a plant. In addition, since the above material canbe processed into various shapes by using a light-weight base materialinstead of natural soil, the weight of the planting material can largelybe reduced.

Based on the above effects, the water-retaining support for plant andplant body-growing water-retaining material according to the presentinvention can support or hold a plant at the time of the growth of theplant and can also function as a source for supplying water to theplant. More specifically, these materials can supply water to a plantwithout inhibiting the growth of the plant, when the support is used asa water-retaining support and plant body-growing water-retainingmaterial for fluid seeding, farm cultivation, field cultivation,virescence engineering, etc.

The water-retaining support for plant and plant body-growingwater-retaining material according to the present invention areeffectively usable as a planting material for “pot-type” products suchas cell-type shaped seedlings, community pot seedlings, and pot-typeseedlings, the production and circulation of which have rapidly beenincreased, particularly in protected horticulture under structures. Inaddition, when an inorganic material and an organic syntheticresin/rubber are used as a plant body-growing water-retaining material,such a material can be a completely synthetic material, whereby thepropagation of microbes therein, and the decay thereof can easily becontrolled.

Further, the water-retaining support for plant and plant body-growingwater-retaining material according to the present invention may easilybe formed into a product which is beautiful in its appearance and isclean, as compared with those of the conventional natural supports andwater-retaining materials, and therefore the support and materialaccording to the present invention can effectively be utilized as aplanting material for indoor type plants.

What is claimed is:
 1. A water-retaining support for plants comprising apolymer (A) having a calcium ion absorption of 0-100 mg per 1 g of thedry weight thereof, having a chlorine ion content of 0.07-7 mmol per 1 gof the dry weight thereof and having a water absorption magnification inion-exchange water at 25° C. of 1.0×10¹ to 1.0×10³.
 2. A water-retainingsupport for plants according to claim 1, wherein the polymer (A) is apolymer having a carboxyl group bonded to the polymer chain thereof, andthe content of alkali metal salt or ammonium salt of the carboxyl groupis 0.3 to 7 mmol per 1 g of the dry weight of the support.
 3. Awater-retaining support for plants according to claim 1, wherein thepolymer (A) is a polymer comprising at least 3-17 mmol of a carboxylgroup bonded to the polymer chain thereof per 1 g of the dry weightthereof, and the content of alkali metal salt or ammonium salt of thecarboxyl group is 0.3 to 7 mmol per 1 g of the dry weight of thesupport.
 4. A water-retaining support for plants according to claim 3,wherein the polymer (A) is a polyacrylic acid-type polymer.
 5. Awater-retaining support for plants according to claim 3, which furthercomprises a polyvalent metal salt of a carboxyl group.
 6. Awater-retaining support for plants according to claim 1, wherein thepolymer (A) is surface-crosslinked.
 7. A water-retaining support forplants according to claim 1, wherein the counter ion to the chlorine ionis Na, K, Ca and/or NH₄ ion.
 8. A plant body-growing water-retainingmaterial comprising a mixture of a polymer (A) and a plant body-growingsupport (B), the polymer (A) having a calcium ion absorption of 0-100 mgper 1 g of the dry weight thereof, having a chlorine ion content of0.07-7 mmol per 1 g of the dry weight thereof and having a waterabsorption magnification in ion-exchange water at 25° C. of 10 to 1000.9. A plant body-growing water-retaining material according to claim 8,which has been shaped into a molded product.
 10. A plant body-growingwater-retaining material plant according to claim 8, wherein the polymer(A) is a polymer having a carboxyl group bonded to the polymer chainthereof, and the content of alkali metal salt or ammonium salt of thecarboxyl group is 0.3 to 7 mmol per 1 g of the dry weight thereof.
 11. Aplant body-growing water-retaining material plant according to claim 8,wherein the polymer (A) is a polymer comprising at least 3-17 mmol of acarboxyl group bonded to the polymer chain thereof per 1 g of the dryweight thereof.
 12. A plant body-growing water-retaining material plantaccording to claim 8, wherein the support (B) is a rubber and/or asynthetic resin.
 13. A plant body-growing water-retaining material plantaccording to claim 8, which further comprises a binder (C).
 14. A plantbody-growing water-retaining material plant according to claim 9,wherein the molded product is selected from the group consisting of:pressure-molded pellet-like products; cut or crushed products from thepressure-molded sheet-like material, rod-like material, or block-likematerial; cut or crushed products from the heat-molded sheet-likematerial, rod-like material, or block-like material; and cut or crushedproducts from the foamed sheet-like material, rod-like material, orblock-like material.
 15. A plant body-growing water-retaining materialplant according to claim 9, wherein the molded product is a foamedmolded product.
 16. A water-retaining support for plants according toclaim 4, which further comprises a polyvalent metal salt of a carboxylgroup.
 17. A water-retaining support for plants comprising a polymer (A)comprising at least 3-17 mmol of a carboxyl group bonded to the polymerchain thereof per 1 g of the dry weight thereof, wherein the content ofalkali metal salt or ammonium salt of the carboxyl group is 0.3 to 7mmol per 1 g of the dry weight of the support, the polymer (A) having acalcium ion absorption of 0-100 mg per 1 g of the dry weight thereof,having a chlorine ion content of 0.07-7 mmol per 1 g of the dry weightthereof and having a water absorption magnification in ion-exchangewater at 25° C. of 1.0×10¹ to 1.0×10³.
 18. A water-retaining support forplants according to claim 17, which further comprises a polyvalent metalsalt of a carboxyl group.
 19. A water-retaining support for plantscomprising a polymer (A) having a calcium ion absorption of 0-100 mg per1 g of the dry weight thereof, having a chlorine ion content of 0.07-7mmol per 1 g of the dry weight thereof and having a water absorptionmagnification in ion-exchange water at 25° C. of 1.0×10¹ to 1.0×10³,wherein the counter ion to the chlorine ion is Na, K, Ca and/or NH₄ ion.20. A plant body-growing water-retaining material comprising a mixtureof a polymer (A) and a plant body-growing support (B), the polymer (A)having a calcium ion absorption of 0-100 mg per 1 g of the dry weightthereof, having a chlorine ion content of 0.07-7 mmol per 1 g of the dryweight thereof, having a water absorption magnification in ion-exchangewater at 25° C. of 10 to 1000, and having a carboxyl group bonded to thepolymer chain thereof, wherein the content of alkali metal salt orammonium salt of the carboxyl group is 0.3 to 7 mmol per 1 g of the dryweight thereof.
 21. A plant body-growing water-retaining materialcomprising a mixture of a polymer (A) and a plant body-growing support(B), the polymer (A) being a polymer comprising at least 3-17 mmol of acarboxyl group bonded to the polymer chain thereof per 1 g of the dryweight thereof, having a calcium ion absorption of 0-100 mg per 1 g ofthe dry weight thereof, having a chlorine ion content of 0.07-7 mmol per1 g of the dry weight thereof and having a water absorptionmagnification in ion-exchange water at 25° C. of 10 to
 1000. 22. A plantbody-growing water-retaining material comprising a mixture of a polymer(A) and a plant body-growing support (B) comprising a rubber and/or asynthetic resin, the polymer (A) having a calcium ion absorption of0-100 mg per 1 g of the dry weight thereof, having a chlorine ioncontent of 0.07-7 mmol per 1 g of the dry weight thereof and having awater absorption magnification in ion-exchange water at 25° C. of 10 to1000.
 23. A plant body-growing water-retaining material comprising amixture of a polymer (A), a plant body-growing support (B) and a binder(C), the polymer (A) having a calcium ion absorption of 0-100 mg per 1 gof the dry weight thereof, having a chlorine ion content of 0.07-7 mmolper 1 g of the dry weight thereof and having a water absorptionmagnification in ion-exchange water at 25° C. of 10 to
 1000. 24. Awater-retaining support for plants comprising a polyvalent metal salt ofa carboxyl group and a polyacrylic acid-type polymer (A) having acalcium ion absorption of 0-100 mg per 1 g of the dry weight thereof,having a chlorine ion content of 0.07-7 mmol per 1 g of the dry weightthereof and having a water absorption magnification in ion-exchangewater at 25° C. of 1.0×10¹ to 1.0×10³.